Compounds of the formula ##SPC3##
Wherein n = 1 or 2, Q is H or X and X is Br or Cl are taught in U.S. Pat. No. 3,296,272 to be highly active fungicides. Among such compounds is included the commercial paint film preservative and industrial fungicide, DOWICIL-A-40, having the formula ##SPC4##
U.s. pat. No. 3,415,832 describes and claims the preparation of the preceding compound (and other such compounds) by oxidation of the corresponding alkylthio (sulfide) compounds with chlorine water. The precursor sulfide compounds are prepared by replacement of a 4-halo substituent in a corresponding polyhalopyridine with an --S alkyl group. For example, the 4-propylthio precursor to DOWICIL-A-40, has been made from 2,3,4,5-tetrachloro pyridine, as taught in U.S. Pat. No. 3,364,223.
Several methods are known for the production of the latter compound and for other tetrahalopyridines but experience has shown that each of said methods leaves something to be desired as a commercial process. It has become apparent that a method based directly on less expensive and more readily obtained starting materials, such as perhalopyridines, would be advantageous. Such a method requires selective replacement of a halogen substituent in the 2 (or 6) position by a hydrogen at some stage of the synthesis.
It is known (I. Collins et al, J. Chem. Soc., (c); pp. 167-174 (1971)) that halogens ortho and para to the ring nitrogen in pentahalopyridines are reactive with hydrazine hydrate. Collins et al also reported the formation of 2,3,4,5-tetrachloropyridine when pentachloropyridine-1-oxide is reacted with hydrazine hydrate and attribute this to intra-molecular deoxidation/decomposition of an intermediate product, tetrachloro-6-hydrazinopyridine-1-oxide. However, halogens in the 4-position of perhalopyridines are several fold more reactive with hydrazine than halogens in the 2- (or 6) positions, thus indicating replacement of the 4-halogen with alkylthio before introduction of a hydrazino moiety ortho to the ring nitrogen. A difficulty with this, however, is the well known ease of displacement of alkylthio substituents from aromatic rings by hydrazine. Further, the 4-position in polyhalopyridines is activated to nucleophilic attack by the effect of the halogens present.